The invention pertains to a closed impeller that includes a coated vane. More specifically, the invention pertains to a closed impeller with a coated vane wherein the coating scheme on the coated vane increases the effective life of the closed impeller. The coating scheme does this by improving the erosion resistance and the corrosion resistance of the coated vane without negatively impacting the mechanical performance characteristics of the closed impeller.
In certain environments, pumps, flow control devices, and other articles that are used to move or transport fluids and slurries are subject to the effects of erosive and corrosive fluids and slurries. One exemplary such article is a closed impeller, which typically is a component of a pump or other article useful to move or transport fluids. In many instances, the impact of the fluid and/or slurries via erosion and/or corrosion diminishes the performance of the closed impeller, and hence, the article (e.g., pump) of which the closed impeller is a component.
Problems caused by erosion and/or corrosion are common to many kinds of articles useful for transport and/or movement of fluids and slurries. In an effort to address this problem of erosion and/or corrosion in the context of a variety of applications, diffusion processes such as, for example, nitriding (e.g., solution nitriding) have been used to provide some protection to the components experiencing erosion and/or corrosion. For example, U.S. Pat. No. 5,503,687 to Berns discloses a solution nitriding of stainless steel in the context of high-speed pump gears and impellers. While a process such as solution nitriding has provided some improvement, in some applications there remains a need to provide a way to treat a component such as a closed impeller so as to improve the effective life thereof.
A cladding process has been used in an effort to improve the life of the components. In such a process, a flexible tungsten carbide-cobalt layer is positioned on the critical surfaces and affixed thereto. U.S. Pat. No. 3,743,556 to Breton et al. shows an exemplary cladding process. A flexible tungsten carbide-cobalt cladding is available from ConformaClad, 501 Park East Blvd., New Albany, Ind. 47150. This flexible cladding can be used to protect against fly ash erosion in a draft turbine blade (see ConformaClad brochure entitled “Tennessee Valley Authority”), as well as to protect against wear in an extruder barrel (see Robert Colvin, “Wear-resistant cladding helps compounder overcome problems”, Modem Plastics Worldwide February 2007).
While the cladding process provides acceptable results with regard to erosion resistance and corrosion resistance, there are limitations associated with the cladding process. First, because of the nature of the process, cladding is not especially applicable to hard-to-reach-reach areas since they cannot be accessed to apply the flexible cladding layer. Second, if dimensional tolerances for the component(s) are tight, the cladding process is typically not suitable for use. Because of the dimensionally tight and hard-to-reach structural features of a closed impeller, the use of the cladding process has limited application to a closed impeller, and especially to the vanes of a closed impeller. Therefore, while a cladding process may be suitable for some applications, there remains a need to provide a way to treat a component such as a closed impeller so as to improve of the effective life thereof, especially when the areas needing protection are hard-to-reach and/or require tight dimensional tolerances.
In the context of a closed impeller, the geometry and physical properties of the vanes, as well as other components, can affect the performance of the closed impeller. The nature of a closed impeller mandates a requirement that the adhesion of a coating to the vane be excellent. Poor adhesion of the coating on the vane results in a decrease of the effective life of the closed impeller. It therefore would be advantageous to provide a coating on a closed impeller (and especially the vane of a closed impeller) that has excellent adhesion. It would be especially desirable if the coating scheme was metallurgically bonded to the surface of the substrate of the vane.
The nature of a closed impeller also mandates that the control over the thickness of the coating be very precise. Unintended variations in the coating thickness can result in a loss of dimensional tolerances that can lead to a decrease of operational performance, as well as a decrease in the effective life of the closed impeller. Further, unintended variations in the coating thickness can cause weight imbalances that are detrimental to the operational performance of the closed impeller and which can result in a decrease in the effective life of the closed impeller. These unintended coating thickness variations are due to the current focus on controlling the thickness to a consistent value throughout the component due to the flux irregularities amplified by the complex geometries of the part. It would be of great benefit to provide a coating on a closed impeller (and especially the vane of the closed impeller) that does not possess unintended variations in the thickness of the coating.
Up until now, the requirements for coating adhesion and control over coating thickness have restricted the coating scheme on a vane of a closed impeller to a single coating layer. Yet, some coating processes used to apply a single coating layer have significant drawbacks. A process such as a thermal spray process (e.g., see U.S. Pat. No. 5,385,789 to Rangaswamy et al.) does not apply an optimum coating scheme because the high heat actually distorts the geometry of the components including the vanes of the closed impeller. For similar reasons, a plasma transfer arc process (e.g., U.S. Pat. No. 5,705,786 to Solomon et al.) does not apply an optimum coating scheme because the high heat distorts the geometry of the components including the vanes of the closed impeller. It would thus be desirable to provide a way to treat (e.g., coat) a component such as a closed impeller (and especially the vanes of a closed impeller) so that the coating process does not distort the geometry of the component.
Typical chemical vapor deposition (CVD) techniques are not suitable because the higher deposition temperatures distorts the geometry of the components including the vanes of the closed impeller. It would be desirable to provide a way to treat a component such as a closed impeller without the need to use higher deposition temperatures such as are extant with CVD techniques.
In addition, CVD techniques and PVD techniques do not provide an optimum coating scheme because they are limited in the magnitude of the thickness of the coating scheme. Conventional PVD techniques typically have a coating thickness limitation of about 10 micrometers. Conventional CVD techniques typically have a coating thickness limitation of about 25 micrometers to about 30 micrometers along with a deposition temperature of at least about 800° C. The thickness of the coating on a vane of a closed impeller should be at least about 35 micrometers. Therefore, it would be highly desirable to provide a coating on a closed impeller (and especially the vane of the closed impeller) that exhibits sufficient thickness such as, at least about 35 micrometers.
An unanticipated failure mode of impellers is the premature loss of balance due to accelerated wear at the inlet and outlet regions of the vanes. Simple velocity profiles would suggest that the outlet will experience higher wear due to the higher velocity, but detailed examinations show that the inlet region due to the change in direction of the fluid causes localized accelerated wear. Therefore the profile of the wear resistant coating layer needs to have additional material at this inlet and outlet regions as compared to other locations along the vane.
It becomes apparent that drawbacks exist with the current techniques used to apply a coating scheme to an article like a closed impeller, and especially to apply a coating to a vane of a closed impeller. It would be highly beneficial to provide solutions to these drawbacks.